This invention relates to a method and apparatus for measuring the thickness of a coating. More specifically, this invention relates to a method and apparatus for continuously monitoring the thickness of a gold coating deposited onto a substrate.
In the manufacture of electrical conductors it is common to form a gold plating to enhance conductivity of the wire. The gold plating process involves passing a wire through an electroplating bath from which a thin coat of gold is plated onto the wire usually with a thickness of the order of from one to about 150 microinches.
The gold plating takes place while the wire is moved at a high speed, of the order of 100 to about 300 feet per minute, so that the thickness of the gold coat is difficult to control precisely. Since, for a particular application, a minimum thickness of the gold plating is needed, the actual plating process is set to deposit a gold coating whose thickness is substantially higher than that which is needed normally. In this manner, the gold coated wire can be assured of possessing the minimum acceptable amount of gold plating and fewer rejects occur. When one considers the scarcity and cost of gold, such a plating process is undesirably wasteful.
It would thus be advantageous to maintain control over the gold plating in such a manner that its maximum thickness can be held to within a few percent of a desired minimum thickness. However, such plating control is not readily obtainable with current practices of measuring the gold thickness on a wire, particularly when the wire is moving at a speed of the order of 100 feet per minute through the plating bath.
A variety of factors may affect the thickness of the plating, such as the plating bath solution concentration, the plating current and other well known factors. There are known methods for measuring thin coatings, such as a gold plating, of the order of a few microinches thickness. In one measurement method, a portion of a plated wire is selected and placed in an X-ray spectrometer for analysis. Such an approach involves an off-line measurement whereby the entire previously plated wire may be rejected if the measured gold thickness falls below an acceptable minimum level.
Another known plating thickness measuring technique involves a .beta.-backscatter technique. This process involves the impingement of a beam of electrons on the plated wire whereby some electrons are absorbed and some are backscattered. For a given geometry and source, the intensity of the backscattered electrons provides an indication of the average atomic number of materials in the coating and substrate and the thickness of the coating. This method is inadequate when the atomic numbers of the plating material and the substrate material are close together. In addition, a relatively high background signal is generated regardless of sample thickness thereby limiting the ability of such a method to discriminate when the thickness signal is weak.
X-ray techniques for measuring the thickness of a gold plating are known and involve irradiating a sample with X-rays from an X-ray source or with radiation from an isotope source. The sample responds with the emission of X-rays which are characteristic of the material of the sample.
Either the intensity of the substrate or plating X-ray lines or their ratio may then be a measure of thickness of the coating. The X-ray measuring techniques can be of the wavelength dispersive or energy dispersive kinds. Both techniques are well known and are described extensively in the literature.
In a conventional X-ray wavelength dispersive analysis of the thickness of the gold plating, there exists high sensitivity to the relative position of the wire to the detector as well as to variations in wire size. Such a sensitive response renders the wavelength dispersive X-ray analysis instrument less than fully desirable for monitoring and controlling the thickness of a coating deposited onto a substrate, such as gold plating on a wire, in an on-line application where it is desirable to perform the measurements in real-time on a moving sample. Furthermore, any change in the substrate or coating materials requires a physical change in the position of the X-ray detectors thereby limiting the versatility of such an apparatus.